The invention relates to a process for controlling a hybrid propulsion system for a watercraft, the system comprising at least a combustion engine, at least one electric machine acting as a generator and as a motor, a drive train with a propeller, a battery and a control unit, a clutch being provided between the combustion engine and the electric machine.
Hybrid propulsion systems are used in small watercraft, Yachts and also in bigger ships. Known systems are units comprising a combustion engine with an electric engine, usually operable as a motor or a generator. There can also be a plurality of electric machines. The electric machine is arranged between the combustion engine and the drive train. The clutch enables the combustion engine to be linked to the electric machine, and therefore propulsion by both machines together. A further clutch can be provided between the electric machine and the drive train.
In general, the problem with such propulsion systems is to accord the characteristic load/speed curve with the performance graph of the combustion engine. The characteristic load/speed curve is essentially that of the propeller (in the following the propeller curve), which does not fit well with the performance graph of the combustion engine. It passes throughout in regions of the performance graph with higher to high specific fuel consumption. This renders an efficient operation of the propeller difficult or even impossible. Usually, when selecting a combustion engine and a propeller, only the maximum speed of the vessel is considered.
Such a system is described in WO 2010/115479 A1. The description mentions that travel at low speed is possible either with electric drive only or with drive by the combustion engine. In the latter case, the combustion engine not being sufficiently loaded, the surplus torque can be used for charging the battery via the electric machine operating as a generator. It says further that this is advantageous foremost when travelling at low propulsion load, as this brings the operating point of the combustion engine in its performance graph nearer to the region of low specific fuel consumption. Even then, the operating point is still far away from minimal consumption, the electric machine would have to be heavily oversized. This would not only increase its bulk, the electric machine would also generate a loading current that is too high for the battery and therefore damage the battery. It would also be oversized for electric travel.
The description further states that, when travelling at medium to high speed of the vessel, the combustion engine operates at a comparatively high load and that the operating point of the combustion engine in the characteristic graph is in or near the region of minimum fuel consumption. As nothing is disclosed about the layout of the propeller, it is most likely to be conventional and the operating point is at best somehow nearer the region of minimal fuel consumption, but not in it.
With conventional layout of the propeller, its characteristic load curve intersects the load hyperbola in the point of rated power. If the characteristic load curve of the propeller passes lower, the engine would overspeed. If it passes higher, the vessel would not reach the speed corresponding to the power of the engine, because the propeller only reaches a lower number of revolutions at that torque. The conventional layout is therefore considered as optimal and thus generally accepted.
What is not considered in this divulgation is, that vessels are far from being operated in the operating point of rated power of the combustion engine and that the overall efficiency of the propulsion system for all operating states (which means over a realistic load collective) also depends on the efficiency at which the electric current is generated.
With the foregoing in mind, it is the object of the invention to minimize fuel consumption and emissions of a specific propulsion system for all states of operation, and possibly also to increase the lifetime of the combustion engine.